The First Animals

THE FIRST ANIMALS

According to molecular clock data, the separation of the Animal Kingdom from the Plant Kingdom occurred 1609±60 Ma (2004HedgesSB_ShoeJL). Molecular estimates indicate that the last common ancestor of multicellular representatives of the fauna (Metazoa) was Neoproterozoic, most likely younger than 800 million years (2015PisaniD_LiuAG). Biomarkers of metazoan living systems identified in sedimentary deposits indicate their presence in the seas since the Cryogenian period (2018ZumbergeJA_SummonsRE), which began around approximately 720 Ma (2023CohenKM_CarN).

Multicellular animals were part of early Ediacaran ecosystems and are found below layers dated to 632.5 ± 0.5 Ma (2007Yin_LHuJ). The oldest fossil of a multicellular organism resembling a sponge (Porifera) is about 600 Ma (2015YinZ_TafforeauP). The last Neoproterozoic glaciation (about 582 Ma) coincides with the appearance of complex organisms in the fossil record (2007BowringSA_AllenPA). Evidence of burrowing by any animal larger than 1 cm is absent from rock layers formed before 575 Ma (2002ErwinDH_DavidsonEH). A diverse assemblage of fossilized bodies and tracks of bilateral organisms (Bilateria) have been found in deposits ranging in age from 558±1.0 to 555.3±0.3 Ma (2000MartinMW_KirschvinkJL; 2003GrazhdankinDV). The oldest protostomes (Protostomia) like mollusks with shells were given the name Kimberella (2007FedonkinMA_IvantsovAY).

Around 543 Ma, a complex community of soft-bodied bilaterians appears in the fossil record (1995GrotzingerJP_KaufmanAJ). The tracks of the first burrowing creations of the worm Treptichnus pedum are dated to the period 542-541 Ma (2007BowringSA_AllenPA). Perhaps 540 Ma, Saccorhytus coronarius, the founder of the line of deuterostomes (Deuterostomia), saw the light of day (2017HanJ_HuangH). In the group of bilaterally symmetrical animals, the phylum Chordates (1995ChenJY_ZhouGQ) emerged 525 Ma. This lineage laid the foundations for the creation of an internal skeleton and joints, in one of which ligamentum capitis femoris (LCF) was later formed.

Upper Vendian, Mezen Formation (about 555 Ma). Slab with imprints of transversely segmented organisms from the extinct type Proarticulata, showing traces of movement and feeding (fragment). Exposition of the Orlov Paleontological Museum (Moscow); photo by the author.

Ovatoscutum concentricum Glaessner et Wade. Vendian, Winter Coast of the White Sea. Collection of the Orlov Paleontological Museum (Moscow); photo by the author.

Brachiopods. Collection of the Orlov Paleontological Museum (Moscow); photo by the author.

Scheme of structure (A) and reconstruction (B) of the most ancient chordate Haikouella, early Cambrian, approx. 539 million years. Exposition of the Orlov Paleontological Museum (Moscow); photo by the author.

References

Hedges SB, Blair JE, Venturi ML, Shoe JL. A molecular timescale of eukaryote evolution and the rise of complex multicellular life. BMC evolutionary biology. 2004;4(1)1-9. [link.springer.com]

Pisani D, Liu AG. Animal evolution: only rocks can set the clock. Current Biology. 2015;25(22)R1079-81. [cell.com]

Zumberge JA, Love GD, Cárdenas P, Sperling EA, Gunasekera S, Rohrssen M, Grosjean E, Grotzinger JP, Summons RE. Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals. Nature Ecology & Evolution. 2018;2(11)1709-14. [nature.com]

Cohen KM, Harper DAT, Gibbard PL, Car N. The International Commission on Stratigraphy (ICS) International Chronostratigraphic Chart. September 2023. [stratigraphy.org]

Yin L, Zhu M, Knoll AH, Yuan X, Zhang J, Hu J. Doushantuo embryos preserved inside diapause egg cysts. Nature. 2007;446(7136)661-3. [ui.adsabs.harvard.edu , academia.edu]

Yin Z, Zhu M, Davidson EH, Bottjer DJ, Zhao F, Tafforeau P. Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian. Proceedings of the National Academy of Sciences. 2015;112(12)E1453-60. [pnas.org]

Bowring SA, Grotzinger JP, Condon DJ, Ramezani J, Newall MJ, Allen PA. Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman. American Journal of Science. 2007;307(10)1097-145. [ajsonline.org]

Erwin DH, Davidson EH. The last common bilaterian ancestor. Development. 2002;129(13)3021-32. [journals.biologists.com]

Martin MW, Grazhdankin DV, Bowring SA, Evans DA, Fedonkin MA, Kirschvink JL. Age of Neoproterozoic bilaterian body and trace fossils, White Sea, Russia: implications for metazoan evolution. Science. 2000;288:841-5. [science.org]

Гражданкин ДВ. Строение и условия осадконакопления вендского комплекса в Юго-Восточном Беломорье. Стратиграфия. Геологическая корреляция. 2003;11(4)3-23. [repository.geologyscience.ru]

Fedonkin MA, Simonetta A, Ivantsov AY. New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications. Geological Society, London, Special Publications. 2007;286(1)157-79. [lyellcollection.org]

Grotzinger JP, Bowring SA, Saylor BZ, Kaufman AJ. Biostratigraphic and geochronological constraints on early animal evolution. Science. 1995;270:598-604. [science.org]

Han J, Morris SC, Ou Q, Shu D, Huang H. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature. 2017;542(7640)228-31. [nature.com]

Chen JY, Dzik J, Edgecombe GD, Ramsköld L, Zhou GQ. A possible Early Cambrian chordate. Nature. 1995;377(6551)720-2. [nature.com]

The original text in Russian is available at the link: Первые животные

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